I. Field of the Invention
The present invention relates generally to an electronic circuit for protecting an electrical load from damage due to voltage drops or current surges applied to the load, and more particularly, to such a circuit with a condition responsive switch which is activated only during zero-crossings of the AC line driving load.
II. Description of the Prior Art
Commonly known household appliances often include an electric motor which is powered by conventional AC current from a standard power outlet. These motors are constructed for operation at a particular rated voltage and current. Although the rated voltage and current is generally supplied to the motor by the standard power source, abnormal conditions can occur which significantly drop the voltage applied to the load or which produce a current surge to the load. These changes in voltage and current applied to the load can severely damage the windings and other components of the electrical motor, especially as a result of the overheating which occurs during these changes. Further damage is also caused from abnormally high instantaneous voltage spikes which can be imposed upon a power line by lightning.
One previously known device for protecting a load from abnormal voltage or current conditions is to use a thermally activated circuit breaker or fuse. Typically, a fuse comprises a very thin conductor which breaks apart when heated as abnormally high currents pass through the conductor and thus, these devices are quite simple, inexpensive, and small. On the other hand, the circuit breakers comprise a simple switch which opens when heated by the high current. Nevertheless, both of these devices are disadvantageous because they do not respond quickly enough to the change in circuit parameters until after the abnormal condition has occurred and been applied to the load for a short time. Moreover, once the fuse has opened, it is destroyed and must be replaced. The circuit breaker must be manually reset to its closed position. Thus, neither of these devices recloses the circuit automatically.
Another previously known form of load protector comprises the use of a switching device or switching circuit which is activated by a sensor used to detect abnormal voltages or currents applied to the load. These previously known switching devices are disadvantageous for the reason that they can induce radio frequency interference when the switch is turned on and off. Moreover, the switches are typically made responsive to a single circuit condition such as voltage changes or current changes, but not both. For example, in U.S. Pat. No. 3,657,603 to Adams, a relay control is disclosed which is responsive to only overvoltage and undervoltage conditions of the line voltage applied to the load. In addition, other previously known switching circuits are often quite complex and thus expensive to produce. Furthermore, they often utilize relay coils which substantially increase the physical size of the circuit such that miniaturization incident to the use of microelectronics is not possible with these circuits.